The invention relates to a method and a plant for reducing the nitrogen oxide (NO.sub.x) emission from a kiln plant for burning mineral raw materials, e.g. cement raw materials, which plant comprises at least two separate, multistage preheater strings, the one comprising a separate precalcining zone with a separate fuel inlet, whereas the other(s) is (are)--as seen in the raw material feed direction through the plant--coupled immediately before the kiln and connected to the latter by a kiln riser pipe, and in which plant air/smoke gas is carried through each separate string by means of a fan mounted in each string, the string with the precalcining zone being provided with air in the form of spent cooler air from a material cooler mounted after the kiln, whereas the kiln string(s) is (are) provided with hot smoke gas directly from the kiln.
Depending on the fuel type used a certain amount of nitrogen oxide (NO) is generated in such plants, viz. in the kiln burning zone, in the kiln riser pipe (if firing is effected herein) and in the separate precalcining zone, respectively, and in order to comply with the requirements for limiting air pollution, which are becoming increasingly restrictive in the industrialized countries, it will be necessary to take measures i.a. to limit the NO.sub.x -emission from the plants in question.
The NO.sub.x -emission consists primarily of the simple compound NO which, according to its origin, may be divided into two categories:
1. Thermal NO the nitrogen atom of which originates from the N.sub.2 content of the combustion air. N.sub.2 which comprises two nitrogen atoms is a very stable compound which is only split at very high temperatures. Therefore thermal NO is almost exclusively formed in the burning zone of a kiln and only to a small degree in a separate calciner.
2. Fuel NO where the nitrogen atom originates from the fuel. Coal, a commonly used fuel in the kiln plants described above, typically contains 1-1.5 weight % of nitrogen. Heavy fuel oil typically 0.4%, petro coke considerably more.
The NO formed in a separate calciner of a kiln plant is primarily fuel NO.
If coal is used as fuel and is consequently subjected to closer chemical examination it is found that the nitrogen atoms are always positioned separately and scattered in the very large molecules of the coal. This is the reason why, to a very large extent, the nitrogen atoms convert to NO during combustion of the coal as the nitrogen atoms, having been positioned in pairs, would otherwise be emitted in the form of N.sub.2 during the combustion.
In the light of this it might reasonably be assumed that for each nitrogen atom N contained in the fuel an NO-molecule would be produced. However, this is not the case, in that "only" 20-50% of the added N atoms are converted into NO-molecules during e.g. calciner firing.
This is due to the N-atoms not being removed directly from the solid coal by oxygen as this would mean a 100% conversion. The N-atoms would appear to form a part of small molecules or radicals detached from the solid coal. The N-atoms transported in this way reacts with oxygen thereby forming NO but, likewise, they react readily with NO, if present, thereby forming N.sub.2. such N.sub.2, however, cannot be split again at the temperatures prevailing in a calciner.
Consequently, one way of explaining it is that N-atoms, when removed as gas, may take two routes:
I. They may react with O.sub.2 thereby forming NO. PA1 II. They may react with NO thereby forming N.sub.2.
On this basis it would be obvious to try to remove all oxygen present during the entire process, i.e. by burning at a low oxygen excess thereby stopping process (I), but, of course, such an approach is not feasible, the point being that the coal should burn. When consequently a certain amount of NO is produced in process (I) a subsequent process (II) may also take its course thereby removing a considerable part of the NO.
It will appear that the factor which in a kiln riser pipe removes some of the thermal NO produced in large amounts in the burning zone of a kiln is not the low excess air factor as such, but the addition of a nitrogen reducing agent, e.g. an N-containing fuel, in combination with the near-complete absence of O.sub.2. Moreover the NO-removal occurring will be directly proportional to the fuel fed to the riser pipe due to a one-to-one combination of NO and N-atoms from the fuel.
If, accordingly, the problem associated with the attempts to reduce the NO-production is considered for a separate calciner, the solution suggested by the hitherto known technique would be to establish low excess air conditions within the calciner. This, however, would concurrently cause the combustion of the fuel, e.g. coal, in the calciner to be discontinued and therefore this solution has never been employed.
U.S. Pat. No. 4,080,218 discloses a singlestring kiln plant wherein the riser pipe mounted immediately after the kiln is constructed as a combined precalciner and NO.sub.x -reducing zone which is at the bottom delimited by an inlet for supplying an NO.sub.x -reducing agent, e.g. coal, positioned at a level approximately identical to that of the inlet for preheated raw materials from the penultimate preheater stage, and it is at the top delimited by an inlet for combustion air or tertiary air in the form of spent cooler air, and in which plant a separator cyclone after the riser pipe may be provided with an air inlet to ensure sufficient combustion air for carrying out the precalcining which is effected partly in the riser pipe partly in the cyclone. It naturally follows that this combined solution consisting in having an NO.sub.x -reducing zone and a precalciner in one and the same plant unit, viz. the kiln riser pipe, is not readily applicable for multistring plants where it is necessary to carry out the NO.sub.x -reduction separately in the kiln string and the calciner string, respectively.
German publication No. DE 3100661 discloses another method and a single- or two-string plant for the reduction of the NO.sub.x -emission from a rotary kiln plant also comprising a precalciner. According to this publication it is attempted to apply the method known from previous singlestring kiln systems for NO.sub.x -reduction by establishing a low oxygen excess in the lowermost part of a precalcining zone, whereto the kiln smoke is also fed, and then add the lacking air at a later stage of the precalcining zone thereby establishing normal air excess conditions in the uppermost part of the zone, into a plant in which the precalcining process is divided into a real "precalcining stage" followed by a so-called "post-calcining stage".
Finally, DE 3.522.883 discloses a singlestring kiln plant having a separate calciner wherein an NO.sub.x -reducing zone is established in the kiln riser pipe by means of a fuel firing and a tertiary air supply, but where, similarly to the plant according to DE 3,100,661, a separate precalcining zone is provided, and where special control means control the fuel and raw material supply to the NO.sub.x -reducing zone depending on the NO.sub.x -reduction obtainable with the added amount of tertiary air.
However, the drawbacks associated with the two latter known methods consist in that only a limited amount of fuel--in practice 10-15% of the total fuel supply to the kiln installation--can be fired into the kiln riser pipe for breaking down the NO produced in the kiln burning zone in the feeding of air to the top of the riser pipe to ensure a complete combustion in the lowermost or the penultimate preheating stage in the kiln string of unburned components in the exhaust gas. Since, however, this amount of air has to be limited to avoid an undesirable increase in the smoke gas temperature during a continued combustion further up in the preheater string and a resulting unwanted increase in the energy consumption of the plant, this amount of air automatically limits the amount of fuel which could be fed into the kiln riser pipe and consequently the NO.sub.x -reduction obtainable in the latter.
The following are the findings of a calculated example based on information supplied by the applicant of DE 31100661 about the distribution of a given amount of N-containing fuel of 140 kg coal/ton of clinker to a two-string plant in feeding 40% of the coal to the rotary kiln, 10% to the kiln riser pipe and 50% to the calciner:
Without any firing into the riser pipe, about 40% of the fuel supplied to the plant would be fed to the kiln and about 60% to the calciner, which on the terms given would result in an emission of 1 kg NO/ton of clinker from the kiln and 0.4 kg NO/ton of clinker from the calciner. Since, however, a 10% fuel supply is fed to the riser pipe about 0.1 kg NO/ton of clinker is broken down therein due to the amount of N added. Thus the kiln string emits 0.90 kg NO/ton of clinker. The calciner's fuel-NO production is slightly lower, only 50% compared to the normal 60% being fired to the calciner. The NO-emission from the latter is consequently 50/60.times.0.4 kg NO/ton of clinker=0.33 kg NO/ton of clinker and the total NO-emission from the plant amounts to 0.90+0.33=1.23 kg NO/ton of clinker as compared to an emission without any reduction of 1+0.4=1.40 kg NO/ton of clinker, i.e. a fairly modest reduction. A substantial reason for this is that, in practice, it is only possible in such a plant to add about 10% of the total fuel supply to the kiln riser pipe, a factor which sets the limit to the obtainable NO.sub.x -reduction.